Since the 1960s, public health authorities have advised people to avoid saturated or animal fats like butter or lard in favour of unsaturated vegetable oils for cholesterol-lowering benefits. This week, researchers offered a fresh analysis of data from the Sydney Diet Heart Study, which studied 458 Australian men aged 30 to 59 with a history of cardiovascular disease for more than three years.

Our bodies may not treat all vegetable oils the same. (Dario Pignatelli/Reuters)

"The group that was randomized to omega-6 from safflower oil, they had increased risk of death from all causes as well as death due to coronary heart disease and death due to cardiovascular disease and this was despite significant cholesterol lowering," said study author Dr. Christopher Ramsden, a clinical investigator with the U.S. National Institutes of Health in Washington.

Half the participants were told to consume linoleic acid — an unsaturated omega-6 fatty acid — in the form of safflower oil and safflower oil polyunsaturated margarine.

Ramsden speculated that having more omega-6 in the diet could promote oxidation and inflammation in the arteries.

The findings add to the evidence of how polyunsaturated vegetable oils aren't all the same, said nutrition professor Richard Bazinet, who studies fatty acids at the University of Toronto.

"People just ran with the ball that polyunsaturates are good for you, so therefore each individual one must be good for you. That turns out not to be true," Bazinet said.

Health Canada has given the nod to the vegetable oil industry to allow companies to put new health messages on their labels saying that replacing saturated fats with unsaturated fats including omega-6 vegetable oils help lower cholesterol.

Bazinet said this week's study in the British Medical Journal suggesting that omega-6s may borderline increase heart attack risk should be enough to trigger a re-evaluation of Health Canada's approval of the claims.

"There's absolutely no evidence to suggest that we should increase more of it, when we have the American Heart Association and a potential health claim in Canada saying yes, this is something we should consume more of, it's absolutely backwards with the evidence," he said.

"They've got to re-evaluate that."

Health Canada turned down a request from CBC News to talk about the study, saying it will review the research.

Bazinet said policymakers should wait for better evidence before allowing claims.

The findings also challenge the American Heart Association's recommendation that people get more omega-6, which the group said it stands by. In contrast, in the U.K. and France, people are advised to lower intake of omega-6, said Dr. Artemis Simopoulos, president of the Centre for Genetics, Nutrition and Health in Washington.

The goal should be to consume a balanced amount of omega-6 and omega-3 fatty acids, she advised Canadians and Americans.

"We should lower the intake of omega-6 rich oils such as corn oil, sunflower, safflower, cottonseed oil, including soybean," Simopoulos said. "Increase the amount of omega-3 fatty acids in our diet, which can be obtained from oils that are rich in omega-3s such as flaxseed oil, canola oil."

Consumers shouldn't be left with the message from a health claim that omega-6 fatty acids lower the risk of heart disease, Simopoulos stressed in calling the approval of the health claim in Canada a mistake.

In the Australian study, men got about 15 per cent of their total calories from omega-6. In comparison, Canadians currently get about seven per cent of their intake from omega-6 fatty acids, Bazinet said.

Dr. Dariush Mozaffarian of Harvard University's School of Public Health in Boston studies dietary habits and cardiovascular disease. Mozaffarian criticized the 40-year-old Australian study, saying the margarine used to increase intake of omega-6s was also high in trans fat.

"Some of this effect, the harm, could be clearly due to the fact that in the 1970s when this trial was done, people didn't know about the harms of trans fats."

For consumers, the subtle and shifting messages can be confusing. Bazinet advises reading labels carefully, whether it be on oils, salad dressings or processed foods. Like Simopoulos, he said flax and canola are the safer options and people might want to stay away from the straight safflower, corn and sunflower oils that typically make up five per cent of purchases.

In response to the story on CBC-TV's The National, the Vegetable Oil Industry of Canada said Monday that it stands by the research submitted to Health Canada in support of its claim. The industry group pointed to literature it said shows that a one per cent drop in cholesterol reduces the risk of heart disease by about two per cent.